Blocker of geomagnetically induced currents (GIC)

ABSTRACT

A device for mitigating Geomagnetically Ground Induced Currents (GIC) flowing from the neutral of a star-connected three-phase power transformer to ground without compromising its basic insulation, operation or integrity. Resistor (R 10 ), of an adaptive non-linear negative volt-ampere characteristic is inserted, from the transformer neutral (N) to ground (G), by opening the ground switch (S 1 ) depending on its DC flow magnitude. Such an insertion to render said induced currents, for any given GMD severity, blocked and inconsequential to the equipment because of (R 10 ) very large resistance prevailing at that condition. Conversely, under a hypothetical and rare simultaneous large power system fault currents, (R 10 ) non-linear negative volt-ampere characteristic becomes very low ohmic and thus an expeditious protective bolted path to ground; all process formulated/designed according to IEEE/ANSI/NEMA recommended insulation coordination guidelines in order to sustain proper protective margins to the equipment neutral basic insulation levels (BIL/BSL).

TECHNICAL FIELD

This invention relates to a scheme for blocking Geomagnetically InducedCurrents (GIC) in a transformer apparatus of an AC power system by meansof a passive device connected to the neutral end of a winding of suchapparatus, part of a GIC circuit.

BACKGROUND INFORMATION

In its most common form, GIC is a phenomenon that takes place when highmagnetic fields produced by electric particles emanated from the sun,periodically impinging our planet interact with the conductors oftransmission and distribution circuits. Such interaction causes,according to the laws of physics, the induction of currents in thesecircuits. GIC can therefore flow in the network, the paper by Pirjola,R., Geomagnetically induced currents during magnetic storms; IEEETransactions on Plasma Science, Volume 28, Issue 6, and December 2000Page(s): 1867-1873, is a good reference on this matter. Furthermore thecurrent flow reaches the power transformers as well as the instrumenttransformers, shunt reactors and phase shifters connected to thetransmission lines, entering through their phase connections andreturning from their earthed neutral. The most important effects arerelated to the saturation of those apparatus' magnetic circuitry. Ingeneral it may cause wave distortion and equipment overheating. Possibleoutcomes of this disturbance are the malfunction of protective systemsand/or failure as well as a deterioration of the grid's performance,including voltage collapse; the paper by Bozoki, B. (chair) et al.(Working group K-11, IEEE power system relaying committee); The effectsof GIC on protective relaying, IEEE Trans. Power Delivery, vol 11,725-739, 1996, refers to this issue. A comprehensive document from theEMP Commission of Congress: Report of the Commission to Assess theThreat to The US from Electromagnetic Pulse Attack-Critical NationalInfrastructures, April 2008, makes a full discussion of these matters.At the end of Chapter 2, under Recommendations, the group concludes that‘EMP attack on the electrical power system is an extraordinarily seriousproblem but one that can be reduced below the level of a catastrophicnational consequence through focused effort coordinated between industryand government’. The present invention deals with a mitigationtechnology in order to block the aforementioned GIC without the use ofcondensers. On the other hand, it is clear today from searching relevantprior art, there are basically only two patent documents to be found ascountermeasures for dealing with these perils i.e. U.S. Pat. No.7,589,943 and U.S. Pat. No. 8,035,935, respectively. In both cases alinear resistor is the basic component to reduce GIC; moreover aprotective Varistor MOV (or surge arrester) is devised in parallel withthe resistor to get a by-pass of potential power-system ground currents.While such an approach has widely proven to be cost-effective, actuallyrecommended by the aforementioned EMP Commission, some majortechnological progress taking place in recent time does afford atranscendental additional refinement; indeed state-of-the-art surgearresters, both for AC and, particularly for DC, plus Hall-effect basedmonitoring/relaying instruments are the pillars of this invention. Inthe case of surge arresters a new major short-circuit capability hasbeen established and recognized by the IEEE; IEEE Standard C62.11a™-2008(Amendment to IEEE C62.11a™-2005) eliminates sub-clauses 8.16, 8.17 and8.18 (Pressure Relief) replacing them with 8.21, whereas Short CircuitTesting is established instead; Short-Circuit Ratings now appear inupdated top manufacturers catalogs. On this rooting, surge arresters,based on their continuous negative volt-ampere characteristic arecapable of sustaining a transformer solid neutral grounding under allconceivable system conditions; in other words the provisions of IEEEC62.92 Standard, Guide for the Application of Neutral Grounding inElectrical Utility Systems are key; this normative sets thepositive-sequence to zero-sequence ratios of X₀/X₁≦3 and R₀/X₁≦1respectively, which now get rigorously met in this invention. In fact,the typical resistance range for their non-linear characteristic expandsfrom Mega ohms to fractions of one ohm depending on voltage/current;therefore under normal operating conditions such a value is very large,affording a neutral GIC current blocking should the need arise, whilefor ground faults or transients overvoltages it becomes very small andthus a protecting expeditious bolted connection to earth. In addition,novel Hall-effect Current Transducers (CT) provide both a reliable andaccurate quasi-DC detection sparing any need for the traditional ACconvoluted second harmonic GIC signature indirect detection, given thefact that DC monitoring is not possible through conventional AC currenttransformers (CT), as it has been the case up to now. In sum, by takingadvantage of these developments the role of a main linear resistor, inparallel with a MOV as in the preceding inventions, can be minimized orplainly obviated in dealing with GIC currents. Consequently, for thepurpose of this application such linear resistor is eliminated from thedevice; a major breakthrough considering such a linear resistorrepresents typically 95% of size, cost and device design challenge.

SUMMARY

This invention relates to a scheme for mitigating Ground-InducedCurrents (GIC) from a Geomagnetic Disturbance (GMD) in an electric powertransformer by means of a non-linear negative volt-ampere characteristicresistor designed to block the circulation of such currents. Thepreferred embodiment is based on a simple surge arrester connected fromthe transformer neutral to ground. Therefore this element passivedevice, a non-linear resistor, is inserted in series to a circuit madeprimarily of a transmission lines and a transformer winding with neutralearthed return. Current division, affecting all transmission lines andtransformers involved, can basically block the GMD surge into thespecific transformer winding to be protected according to basic circuittheory. A mitigation philosophy can be established where a GIC currentactual blocking by direct application of Ohm's law is decisivelyaccomplished before a sudden major pulse disturbance.

OBJECTS AND ADVANTAGES

Consequently, it is an object of this invention to provide an externalneutral grounding device to cope with the GIC problem, as defined,simply by applying effectively/carefully basic electricity laws to blocka circuit current; this result must be accomplished for a quasipermanent operation without any limitation or infringement of equipmentstandards, insulation guidelines, and particularly on the transformergrounding code. Furthermore, as known from symmetrical component theory,a neutral resistor is three-times more effective than phase ones tolimit ground currents, for they get amplified by a factor of three withrespect to the corresponding phase ones (the zero sequence resistancesums three times the neutral one); moreover in a system designedbasically to be lossless, i.e. having very low resistances, GIC currentsbecome sensitive to a neutral resistor insertion blocking component.

Nevertheless the basically lossless qualifier remains, since in balancedthree-phase systems there is no steady-state flow through the neutral.In addition neutral-to-ground circuit elements do not get any majorpower duty, neither bear high voltage. It is also an advantage of thisinvention to consist of only two inexpensive standard components of theDistribution Class. Moreover it becomes yet a novel advantageoustransformer neutral grounding means; a traditional utility practice fora host of other applications.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawing.

DESCRIPTION FIG. 1

FIG. 1 depicts a one-line diagram of a circuit embodying the presentinvention as it applies to mitigate GIC currents on a transformer. FIG.1 also depicts two transformer neutral grounding branches: one having anormally-closed ground switch, and a second branch having a non-linearresistor (or surge arrester) R10, intended while inserted, to limit suchcurrents

OPERATION FIG. 1

Regarding FIG. 1, it shows a normally closed ground switch S1, operatedon the basis of its constantly measured DC current magnitude; when suchmagnitude exceeds a given reference, switch S1 opens inserting resistorR10; such resistive component, with its non-linear negative volt-amperecharacteristic is specified according to a given design criteria to havea high blocking resistance before GIC currents flowing from thetransformer neutral (N) to ground. Furthermore such resistor, wheninserted becomes, for contingent ground currents or transientovervoltages, a low resistance bolted path to ground. R10 specificationand functionality are furthermore set by well-established insulationcoordination engineering practice with the transformer's windingneutral-end Basic Insulation Level (BIL) and Basic Switching ImpulseLevel (BSL); this in order to provide proper protective marginsfollowing IEEE, ANSI, and NEMA guidelines. These features would enablethis proposed device to be connected to the transformer neutral in aquasi-steady state mode; yet such a method is not a requisite since DCaccurate detection by means of Hall CT allows reliable insertions asrequired.

CONCLUSION, RAMIFICATIONS AND SCOPE

It should be apparent the present invention adds a major functionalityto surge arresters becoming a very cost-effective means forcounteracting potentially severe GIC currents in a power transformer.Furthermore it yields the following objects and advantages:

-   -   it is simple;    -   it uses minimal transformer plant space;    -   it is a passive device;    -   it is cost effective;    -   it is energy efficient;    -   it is a resistive component can be trouble-free switched on/off;    -   it comprises only two standard distribution voltage components;    -   it implies a negligible layout change to the transformer        substation.

Thus while the preferred embodiments of the invention have beenillustrated herein, it is to be understood that changes and variationsmay be made by those skilled in the art without departing from thespirit and scope of appending claims.

I claim:
 1. A passive external resistive neutral grounding device tappedat the neutral of a star connected three phase winding of a powertransformer to protect it by substantially blocking the flow of harmfulGround Induced Currents from a Geomagnetic or Electromagnetic PulseDisturbance through said transformer without compromising saidtransformer neutral end insulation operation or integrity.
 2. The deviceof claim 1 that comprises a resistor of an adaptive negative volt-amperecharacteristic yielding accordingly a very large ohmic resistance fromneutral to ground in response to said Ground Induced Currents withintended flow through said transformer so as to substantially block theflow of said Currents said resistor yielding alternatively a very lowohmic resistance from neutral to ground in response to large AC groundfault currents providing thus an expeditious bolted conduction to groundas per recommended insulation coordination guidelines designed tosustain proper protective voltage margins to said transformer neutralbasic insulation levels.
 3. The device of claim 1 comprising a normallyclosed ground switch connected from said neutral to ground for solidgrounding or alternatively open for resistor insertion.
 4. A resistiveneutral grounding device to substantially impede Ground Induced Currentsfrom a Geomagnetic or Electromagnetic Pulse Disturbance from flowing ona star connected three phase winding of a power transformer to groundwithout compromising its neutral end insulation comprising an adaptivenegative volt-ampere characteristic resistor inserted from thetransformer neutral to ground sized to substantially block the flow ofsaid Currents the adaptive negative volt-ampere resistance of saidneutral grounding resistor provides alternatively a path to ground ofsubstantially very low resistance as prevailing by virtue of itsnonlinearity under large power system ground fault currents according torecommended insulation coordination guidelines in order to sustainproper protective margins to the equipment neutral basic insulationlevels.
 5. A passive neutral grounding device to substantially blockGround Induced Currents on a star connected three phase winding of atransformer comprising a non-linear resistor having a selected constantohmic value for the range of said Currents such that it substantiallyblocks the flow of said Currents without compromising said transformerneutral end insulation operation or integrity.
 6. The neutral groundingdevice of claim 5 whereas said non-linear resistor provides a boltedby-pass to ground of substantially very low resistance for the range ofAC ground fault currents so as to protect the neutral insulation of saidtransformer.
 7. A neutral grounding device of claim 5 that comprises aresistive means.